support/dependencies: set cmake version min to 3.15

quazip requires cmake 3.15 since version 1.0 and
https://github.com/stachenov/quazip/commit/89e7c201f0215032d347eef6fc0c671a1845738b
https://github.com/stachenov/quazip/commit/818adc82246cefffa2e8851534016cf1db349da7

The rationale for this requirement is that "default locations for the
install(TARGETS command based on the GNUInstallDirs package were only
added in 3.14" and "3.15 is not that much of a difference from 3.14 and
it introduced a lot of useful UI improvements.":
https://github.com/stachenov/quazip/issues/82

Fixes:
 - http://autobuild.buildroot.org/results/5d848a46109aef448ea1d1b857a500d9461dc2d9



Note: we also have some patches to allow some packages to build with
cmake-3.10, and this will not be tenable over the long run.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: add the "note"]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>